The present invention relates to impact modifier materials and methods and apparatus for the production thereof.
Impact modifiers and other additives are commonly introduced into plastics to attain desired properties such as impact strength. Typically, these impact modifiers are core and shell polymers such as methacrylate butadiene styrene (MBS), acrylonitrile butadiene styrene (ABS) or acrylonitrile styrene acrylic (ASA).
Conventionally, these impact modifiers are introduced into the plastics as powder. However, power handling may pose a safety hazard even with the use of special handling equipment to eliminate dust clouds. Fines and longs, which are generally particulates that are prone to become airborne, generally are separated in a cyclone or within a bag filter apparatus. Therefore, attempts have been made to provide impact modifiers in a suitable form for subsequent processing, for example, as pellets.
Impact modifiers have been formed into pellets to facilitate handling. However, commercially available pellets of impact modifiers are expensive, and are deficient in that they have a very wide size distribution range, including fines and longs. For many plastic processing techniques, such airborne prone particulates are unacceptable. Further, fines may bridge and therefore hinder feed when introduced in plastic processing techniques. Additionally, the varying size distributions may segregate, resulting in variations in the plastic composition.
Generally, pellet formation of any material involves extrusion followed by pelletization. During extrusion, the powder is subjected to very high temperatures and pressures. However, formation of impact modifier pellets while retaining the core and shell structure is difficult due to degradation (e.g., scorching) when the material is subjected to the high pressures and temperatures. Impact modifiers having a high rubber cross link density are especially difficult to extrude and subsequently pelletize.
Therefore, it is desirable to provide a form of impact modifiers (i.e., pellets or continuous strands) that retains the core and shell structure during processing. Further, it is desirable to provide forms of impact modifiers that are free-flowing and have narrow size distribution, which generally enhances feeding and compounding of the pellets.
A method for processing an impact modifier material having a core and shell molecular structure, comprises working the impact modifier material in a mild extruder to a state such that at least a portion of the shells of the impact modifier material fuse together to form an impact modifier material output, and pelletizing the impact modifier output.
In another embodiment, a method of pelletizing impact modifier material comprises consolidating the impact modifier material in an extruder; heating the impact modifier material while in the extruder; plasticizing the impact modifier material, wherein the impact modifier material comprises a core and shell structure and wherein the impact modifier material is heated to a temperature less than the melting temperature of the shell of the impact modifier material; and pelletizing the plasticized impact modifier material.
A better understanding of the features and advantages will be obtained by reference to the following detailed description of the invention and accompanying drawings which set forth illustrative embodiments, wherein:
FIG. 1 is a process flow diagram for pelletizing impact modifiers according to an embodiment of the present invention;
FIG. 2 is a schematic of an extruder employed for processing impact modifier material according to an embodiment of the present invention;
FIG. 3 is a cross sectional view of a twin-screw extruder according to another embodiment of the present invention;
FIG. 4 is a schematic of a twin-screw extruder in accordance with one embodiment;
FIG. 5 is a schematic of a twin-screw extruder in accordance with another embodiment.